The invention relates to a method for compensating for measuring error in detecting the currents in an energy storing means in order to ascertain its charge state. The currents that are drawn from the battery or fed into the battery are measured continuously by means of a current measurement. The measured values detected are either integrated or added up over time. The remaining charge in the energy storing means is found on this basis.
German Patent Disclosure DE 4 312 760 A1 discloses a terminal with integrated current measurement. In electronic devices for regulating flows or power levels, current measurement devices are used, as well as input and/or output terminals suitable for corresponding currents. In the prior art, these elements are disposed separately in the device and require a large volume and major expenditure for wiring. By integrating a current meter without a soft magnetic core for guiding the magnetic field, the current sensor can be integrated into the terminal without significantly increasing its structural volume.
To ascertain the remaining charge in an energy storing means, such as a battery, the incoming and outgoing currents are measured by means of current converters. To detect the largest possible measurement range with high precision, two current converters are used. A small current converter is used to detect low currents, while a large converter is used for detecting high currents in the upper measurement range. If the current to be measured exceeds a certain threshold value, then the measurement of the current moves from the first current converter to the second current converter, until the current to be measured drops below the threshold again.
The measurement outcome obtained in this way involves a measuring error, which is composed essentially of two parts, namely a part independent of the measured value (known as the offset) and a part that is dependent on the measured value. The absolute measuring error increases with the size of the current converter. The precision of a current measurement made with a small current converter is therefore greater. The point, the part of the measuring error that is independent of the measured value, that is, the offset, is caused both by the measuring converter and the downstream electronic evaluation unit and is dependent on the temperature. If the remaining charge in an energy storing means is ascertained using two current converters for low and higher currents, it is adulterated in particular by the offset error of the larger current converter, since because of the ensuing integration over time this error is multiplied.
By means of the invention, a permanent calibration takes place of the part of the measuring error, or offset, that is independent of the measured value and is established in the large current converter. As a result, the offset error of the large converter can be compensated for, and more-precise detection of the battery current and thus a more accurate ascertainment of the remaining charge can be brought about.
By the detecting current by both current converters within one measurement range that is common to both converters, a calibration of the converter values obtained can be performed. In this common measurement range of the detecting current, the second converter for the higher currents generates a measuring error, which is dominated by the offset error. By means of the method according to the invention, the outcome of measurement of the second converter for high currents can be calibrated by means of the outcome of measurement obtained simultaneously, and in parallel, by the first converter for lower currents, so that the measurement involving the greater proportion of offset error performed by the second converter for higher currents, which considerably adulterates the overall outcome, can be corrected. The correction of the converter values of the second converter for higher currents is expediently done prior to the integration of the measured values.